U.S. Pat. No. 5,721,587 discloses a revolver punching press having an upper tool revolver and having a lower tool revolver. When processing workpieces during regular operation, punching tools are used that are positioned in mutually opposite tool receiving members of the two tool revolvers and that cooperate with each other. Before the start of the regular workpiece processing operation, one performs a test processing operation. A video camera and a light source of an image capturing device are mounted in two cooperating tool receiving members for receiving punching tools. After the test processing operation, the workpiece with a punched-out portion is moved into the receiving region of the video camera. Subsequently, the video camera records the punched-out portion, which is back-lit by the light source. Finally, the quality of the punched-out portion is examined by computer-supported evaluation of the recording made of the punched-out portion.
The EP publication EP 0 536 685 A1 discloses examining a quality of a perforation of paper webs with a measurement device, which comprises a light source and a detector device spaced apart from each other. The paper web to be examined is moved through the intermediate space between the light source and the detector device. Light directed from the light source to the detector device passes through the perforations of the paper web and reaches the detector device. This produces, owing to the incidence of light, a detector signal that is supplied to an evaluation unit. If irregular conditions prevail when the perforations are produced, for instance, if the perforation blades used are worn, paper fibers remain in the perforations produced. The more paper fibers project into an illuminated perforation, the lower is the intensity of the light which reaches the detector device and the smaller is a signal stroke of the detector signal which is generated at the detector device owing to the incidence of light. Consequently, the signal stroke of the detector signal allows an observation to be made as to whether the perforation of the paper web has or has not been carried out under regular conditions.